Significant background material on the manufacture of lead-acid batteries can be found in the inventor's prior U.S. patent application Ser. No. 11/234,077 and corresponding PCT application PCT/US2005/034214, published as WO 2006/034466 and incorporated herein by this reference. Some of this material is reproduced below.
An important and time-consuming aspect of manufacture of lead-acid batteries is the curing of wet active paste material precursor into a dry porous mass. The paste precursor typically is in the form of flakes of “leady oxide”, i.e. flakes of solidified lead particles which have a PbO coating. The leady oxide is made into a wet, pliable dough (“paste”) by mixing it with water and then with sulfuric acid. The dough then is extruded onto mechanically rigid, electrically conductive grids in a process called “pasting”. The resulting pasted grids are cured at elevated temperature and humidity to react PbO with sulphuric acid, to form lead sulfate salts, and to oxidize the lead core of the leady oxide to PbO to form additional lead sulfate salts.
Lead sulfate salts which provide mechanical strength and porosity to the leady oxide paste, and ultimately become part of the active material, include tribasic lead sulfate 3PbO.PbSO4.H2O (“3BS”) and tetrabasic lead sulfate 4PbO.PbSO4 (“4BS”).
The 3BS typically forms at low temperature and low humidity, whereas 4BS typically forms at higher temperatures (>70° C.) and higher humidity. The 3BS typically forms as small needle-like crystals which measure about 3 microns long and less than about 1 micron in each of width and thickness. The 4BS crystals are larger, and grow in length from several microns to several hundred microns. The longer 4BS crystals have width and thickness in proportion to length. For example, a 300 micron long 4BS crystal might have a width of 60 microns and a thickness of 50 microns. A 4BS crystal that measures 300 microns long by 60 microns wide and 50 microns thick has a surface area of 72000 square microns, and a volume of 900,000 cubic microns. This volume, if tightly packed with the smaller 3BS crystals, would hold about 107 3BS crystals which have a total surface area of about 7.2×106 square microns, i.e. 1000 times greater surface area. The size and shape of the crystals in the cured paste can be measured by scanning electron microscopy (SEM). The amounts of the 3BS and 4BS crystals may be determined by x-ray diffraction (XRD).
In general, the useful capacity of a battery, particularly its ability to deliver high current for short periods of time, as required for example in the starting of internal combustion engines, is improved by increase in the surface area of the lead compounds in the cured paste. Accordingly, large crystals of 4BS are less desirable than smaller crystals, although the presence of 4BS is itself desirable.
The production of 4BS generally requires very careful control of temperature and humidity during cure of the pasted plates. Premature dryout and/or cooling of the plates during the curing process inhibit the formation of 4BS. Some battery manufacturers specify control and uniformity to ±2° C. and ±1% relative humidity (RH) compared to setpoints during cure. It has generally been observed that addition of red lead (Pb3O4) allows adequate processing flexibility to allow production of 4BS over a larger range of temperature and relative humidity relative to setpoints; however, in controlled experiments, the red lead addition appears to interfere with free Pb oxidation.
Production of 4BS entails nucleation of crystals and their subsequent growth. Nucleation is effected by exposing the pasted plates to temperatures of about 70° C. or higher at high humidity at the beginning of cure. Nucleation and crystal growth during cure can require an induction period of about 10 hours since 4BS forms as molecules, which slowly coalesce by diffusion into seeds. These seeds can react with additional nearby material to grow into crystals. The growth rate of the 4BS crystals depends on various factors such as the composition of the lead oxide used, the oxide to sulphuric acid ratio in the paste mixture, the mixer type, mixing time, mixing temperature, temperatures between process steps, flash drying conditions, as well as the temperature and humidity inside the curing chamber, as mentioned above.
The growth of 4BS can proceed by two mechanisms. Large isotropic and “regular”, i.e. uniaxial, crystals, can be prepared by preferential deposition of a material onto one face of a seed crystal by a screw-dislocation or a slip-plane mechanism. Since only one face of each crystal grows, the process is slow. Crystals grown by this mechanism have smooth crystal faces and sharp angles between adjacent faces. Alternatively, crystals that grow anisotropically may be produced faster by fractal growth. Fractal growth entails growing the crystals at many locations and in many different directions simultaneously, i.e. multiaxially. The resulting fractal crystals are irregular and smaller in size. Fractal crystal growth may be confirmed by plotting “quantity produced” vs. time. Regular growth provides a straight-line linear plot. Fractal growth provides a straight-line log-log plot. Fractal crystal growth may produce greater mechanical strength in a paste pellet because the multiaxial crystals interlock better than the uniaxial crystals do. Fractal growth also may produce better electrical conductivity when it occurs in the paste.
Some battery manufacturers prefer 3BS rather than 4BS for engine cranking (also known as SLI, for “starting, lighting, and ignition”) batteries, which may be flooded, gelled, or absorptive glass mat (AGM) in design. To some degree this is because traditional curing processes tend to create highly variable yields of 4BS which have a large crystal size and a small amount of very large pores, and hence low surface area per unit weight. This variability in yield and the undesired crystallinity and porosity tends to cause variable (and generally poor) battery cranking performance in SLI batteries. Accordingly, many manufacturers prefer to avoid formation of 4BS insofar as possible.
When curing conditions are adjusted to preclude nucleation and growth of 4BS, a predominance of 3BS is produced. The 3BS has uniform crystal shape and size (3 microns ×approximately 0.5 microns ×approximately 0.5 microns). When a plate is pasted with 3BS the plate has uniform porosity and high cranking performance.
Historically, free Pb was a desired component of battery paste. The free Pb was thought to generate heat during cure of the pasted battery plates to enhance production of 3BS, 4BS and porosity. This heating, however, was uncontrolled and erratic, and the resultant plates did not always have the composition and/or porosity desired. Free Pb is now considered undesirable. A high amount (more than about 2 wt. %) of free Pb at the end of curing can lead to shedding and spalling failure of the positive plates and/or high self-discharge of “formed” PbO2 plates.
The amount of free Pb in leady oxides typically is about 25 wt. %, but can form in amounts of 20 to 40 wt. % free Pb depending on the apparatus and the process settings. It is difficult and costly to produce a leady oxide with about 15 wt. % or less free Pb, and even more costly to produce a nonleady oxide. This latter usually requires a subsequent thermal processing, in small batches. Discharge capacity of a battery depends on the porosity and surface area of the porous battery electrode. Both the positive electrode, which for a lead-acid battery is the lead dioxide electrode, and the negative electrode, which for the lead-acid battery is the sponge Pb electrode, need porosity. The porosity of negative plates, during battery use, is improved by the well-known use of “expander” additives, which are comprised of barium sulfate, carbon black and lignosulfonic acid salts. All lead-acid electrodes which have a larger surface area have a higher discharge capacity, and higher utilization of the active material at any rate of discharge. In high discharge rate batteries such as SLI batteries, 3BS has been the preferred active material precursor, but if the undesirable growth of large crystals of 4BS can be inhibited, as provided by the invention, 4BS would also be desired in SLI applications. 4BS is the preferred material precursor for deep cycle and long-life stationary batteries. 4BS is also now the preferred precursor for use in modern nonantimonial grid batteries, the so-called “maintenance-free” batteries for SLI, float or cycling application, because the 4BS helps prevent PCL (premature capacity loss), i.e. short battery life. According to the present invention, the best features of 3BS and 4BS can be obtained simultaneously, without any apparent shortcomings, as described below.
Curing promotes adhesion of battery paste to the grids. The battery paste, which has an alkaline pH, reacts with (corrodes) lead alloy in the grid to partially convert the lead alloy to Pb compounds and ultimately to 3BS and 4BS. Generally, the higher the temperature employed during cure, the better the adhesive bond produced.
As mentioned above, production of 4BS depends on nucleation and growth of 4BS crystals. One way to get 4BS nuclei immediately into a battery paste is to use 4BS seed crystals, such as those prepared by grinding large crystals of pure 4BS. Large 4BS crystals can be made by any of several well-known aqueous slurry processes. These processes, however, are slow, and yield only a small amount of 4BS in copious amounts of liquid. Accordingly, this is very costly. Another way to produce 4BS is to use an Eirich mixer wherein 4BS is made into a more concentrated slurry, and then removing excess water by vacuum and heat. A pyrometallurgical reactor (Barton pot) also may be used to make 4BS. A slurry reactor and reactive grinding may be also be used to make 4BS. These methods, however, do not produce multiaxial crystals of 4BS, or seed crystals which can grow multiaxial crystals in battery plate pastes.
Ser. No. 11/234,077 discloses a paste curing additive (“PCA”) for battery paste for use in, for example, lead acid battery positive plates, and its methods of manufacture and use. The PCA limits production of larger 4BS crystals by nucleating growth of numerous 4BS crystals, so that more and therefore smaller 4BS crystals are present in the final product, as well as to grow the 4BS in multiaxial crystal groupings. PCA, which itself contains little 4BS, can be used to make more 4BS, and may be used to reduce the cure time of active material paste, as well as to reduce the amount of energy required during curing.
PCA also may be used to enhance production of 3BS during mixing and cure of battery paste. The PCA may be used to enhance curing of pasted battery plates, especially pasted battery plates intended for SLI lead-acid batteries.
PCA also may be used to achieve greater porosity in the form of higher numbers of pores as well as larger sizes of pores in the cured plate. PCA also may be used to speed oxidation of free lead residue in pasted plates during cure. PCA also may be used to enhance adhesion of the cured paste to the grid. These may provide greater utilization of the active material and easier conversion from the non-active “paste” state to the “active material” state.
PCA in amounts of about 1 wt. % to about 12 wt. % based on the weight of leady oxide may be used to speed the cure of battery plates at temperatures of about 56° C. to about 100° C. at RH of about 10% to about 100%.
Lead acid battery plates which include PCA may also cure faster and may show improved performance. In side-by-side tests for lead-acid traction battery positive plates, PCA outperformed a commercial ground 4BS crystal seed material: <2% free Pb was achieved in <20 hr with PCA, in 24 hr with the competitor material and in >40 hr with no additive (control). When these cured plates were tested in a cycling (charge/discharge) regime, controls operated <1000 cycles, the competitor material operated approximately 1500 cycles and PCA operated to >2500 cycles to end of life. The PCA cells were removed from test to allow testing of other cells; from the data trend it appears that PCA should have operated to >3000 cycles, which is twice the industry standard requirement of 1500 cycles.
The use of PCA may improve development of crystals of lead sulfate such as 3BS and 4BS, and may enhance more rapid development of porosity and the oxidation of free lead.
In one exemplary process, PCA is produced as the reaction product formed by heating a battery paste to a temperature of about 80° C. to about 90° C. for about 5 min. to about 10 min., wherein the battery paste includes sulfuric acid in an amount of about 5 wt. % to about 6 wt. %, water in an amount of about 12 wt. % to about 16 wt. %., and balance leady oxide, all amounts based on total weight of sulfuric acid, water and leady oxide. The additive may be then be used in any of its dried or undried states.
In a second exemplary process, the PCA is produced as the reaction product formed by heating a battery paste to a temperature of about 70° C. to about 90° C. for about 10 min. to about 90 min., wherein the battery paste includes sulfuric acid in an amount of about 3 wt. % to about 10 wt. %, water in an amount of about 10 wt. % to about 20 wt. %., and balance leady oxide, all amounts based on total weight of sulfuric acid, water and leady oxide.
As mentioned above, starting-lighting-ignition (SLI) lead-acid batteries have as their major mission to provide high current to the starter motor, which crank the internal-combustion engines of motor vehicles until they start. There is a market need to improve cranking ability while simultaneously reducing size, weight and cost. Cranking requires a high current (hundreds of amperes) battery discharge over a short period (<1 min.) of time. This requires that the active materials of the battery have a large and intimate contact area: the larger the surface area, the greater the cranking performance, all other things being equal.
Lead-acid batteries contain three electrochemically active ingredients, two being sets of solid members forming numerous opposed pairs of oppositely polarized “plates” and the third being the sulfuric acid electrolyte, which is disposed within and between the plates. The plates are connected together in two groups; the positive polarity plates contain PbO2 and the negative polarity plates contain sponge Pb. As mentioned above, and as further detailed below, plates typically comprise conductive lead alloy frames called “grids” which at manufacture are filled with a “paste” of the approximate consistency of a plastic dough mixed from leady oxide (i.e., flakes of solidified lead particles with a PbO coating), water and sulfuric acid, cured, and then electrochemically “formed” or charged. Pastes intended for positive and negative plates are generally similar, except that pastes for negative plates contain other ingredients called “expanders”. One assembly of two sets of opposite-polarity plates, along with electrically insulating but porous separators, the electrolyte, and the containment for all of these comprise a single cell. Two or more (usually 3 or 6) series-connected cells comprise a battery.
The surface of each plate, both positive and negative, formed by pasting as described above, consists of two intermeshed parts: a discontinuous solid phase formed of the various lead compounds described above, interspersed with a discontinuous void space, which will ultimately be filled with electrolyte. In order to provide adequate surface area, the solid phase should consist of moderately large crystals with a “fuzzy” surface, or more numerous smaller crystals. The latter approach has traditionally been used by battery manufacturers, where the small crystals (approximately 3 microns long) are tribasic lead sulfate (“3BS”) the chemical composition of which is 3PbO.PbSO4 or, if hydrated, 3PbO.PbSO4.H2O. 3BS can be produced during the mixing of the battery paste or during the subsequent recrystallization or “curing” step, typically carried out at <70° C. During mixing and curing, different basic lead sulfate salts can be produced under different conditions. These range from non-basic lead sulfate (0BS, PbSO4) through 1BS (PbO.PbSO4) and 2BS (2PbO.PbSO4) to 3BS as above and ultimately to 4BS (4PbO.PbSO4). 0BS, 1BS and 2BS are generally undesirable in cured battery plates. 4BS, which is nucleated at >70° C. but which can subsequently grow after nucleation at any temperature from nearly 0° C. to 100° C. (below this range H2O freezes and above this range H2O boils, of course), is desirable for use in long-life deep-cycling applications such as fork lift truck batteries, and may be useful to some degree in SLI battery applications as well, if the 4BS crystals can be inhibited from growing overly large. Thus, there is a desire to provide as much 4BS in the battery paste as possible, especially for batteries intended for deep-cycle applications, if certain problems inherent in the use of 4BS as mentioned can be overcome.
More specifically, and as also noted above, while 3BS crystals stop growing at 3 microns, 4BS crystals can grow to become several hundred microns in length. This size is mechanically and electrically desirable, but as these large crystals provide comparatively little surface area, they are electrochemically undesirable. Accordingly, smaller 4BS crystals are desirable to maximize the ratio of surface area to weight.
Generally, one 4BS crystal grows from a single seed, the seed consisting of ground, macro-crystalline 4BS, as is now commercially available. To grow small 4BS crystals, one approach (for example in U.S. Pat. Nos. 7,118,830 and 7,517,370) is to use very finely ground (0.1 to 5 micron) 4BS seeds, but this finely ground material is difficult to handle and provides a significant dust hazard which is undesirable. Another approach is to use a larger quantity of seed (20% rather than the usual 1 to 2%), but seed is costly and larger amounts appear to be preferentially consumed before leady lead oxide in the paste mixing and/or curing processes.
Higher growth temperatures for the growth of 4BS, performed with or without 4BS seeds, impart secondary nucleation of 4BS (desirable) and also generally give better grid-paste adhesion (also desirable) by way of enhanced grid corrosion by the alkaline battery paste. Thus, curing the paste at higher temperatures will generally result in the formation of more 4BS, but manufacturers may not be able or willing to cure the paste at such elevated temperatures.